1. Field of the Invention
This invention relates to electronic switching devices, and particularly to vacuum devices in which electrons are emitted from a cathode by virtue of a field emission process.
2. Description of Related Art
Over the past thirty years, semiconductor device technology has replaced conventional vacuum device technology for all but the most specialised electronic applications. There are many reasons for the preference for semiconductor devices. For example, they are more reliable, they are considerably smaller and they are cheaper to produce than conventional vacuum devices. Furthermore, their power dissipation is much lower than that of thermionic vacuum devices, which require a considerable amount of cathode heating power.
However, in at least one respect vacuum devices are greatly superior to devices based on solid state materials. The vacuum devices are far less affected by exposure to extreme or hostile conditions, such as high and low temperatures. Because the band gaps of useful semiconductors are necessarily of the order of 1 ev and many other interband excitations are lower than this, the excitation of intrinsic carriers is significant and is strongly temperature-dependent at and above room temperature. This severely modifies the characteristics and the performance of semiconductor devices. In addition, the electron occupancy of the traps and other defect states which determine the properties of semiconductor structures is extremely temperature sensitive, particularly at low temperatures. The problems become increasingly acute with the trend towards smaller semiconductor devices and higher integration density.
Vacuum devices, on the other hand, suffer to a much smaller extent from such problems. The density of the conduction electrons which are responsible for thermionic and field emission processes is not dependent on temperature, and because the devices have barriers with large work functions, significant thermal activation requires a temperature of at least 1000xc2x0 K.
However, solid state semiconductor devices can operate at high switching speeds, for example at a switching frequency of, say, 100 GHz. In view of the lower current densities which are achievable in vacuum electronic devices, it is generally accepted that vacuum devices must exhibit lower switching speeds.
It is an object of the present invention to provide a high-speed vacuum switching device.
According to the invention there is provided a vacuum switching device comprising a cathode; extraction electrode means adjacent the cathode for causing electron flow from the cathode; modulation grid means spaced from the cathode and the extraction electrode means for modulating the electron flow; and an anode structure spaced from the modulation grid means for receiving the modulated electron flow.
Further electrodes, such as a collector grid, may be located between the extraction electrode means and the anode structure.